In order to further increase the power and efficiency of gas turbines, the combustion temperatures are raised further. Higher combustion temperatures can lead to a higher content of undesirable NOx in the flue gas. Reducing the NOx content of the flue gas by means of improved combustion technology has, however, limits with respect to mixing, leakage and residency time of the fuel. Further reduction by means of combustion technology does not appear to be possible.
Attention must therefore be directed to how the NOx content of the flue gas can be reduced by methods implemented post-combustion.
It is known, from the article by F. Güthe et al. Flue Gas Recirculation of the Alstom Sequential Gas Turbine Combustor Tested at High Pressure”, Proc. ASME Turbo Expo 2011, Jun. 6-10, 2011, Vancouver, Canada, GT2011-45379, (see FIG. 5 therein), that the NOx content of the flue gas from a gas turbine can be greatly influenced and/or reduced, under certain circumstances, by means of flue gas recirculation (FGR).
The publication US 2009/0284013 A1 discloses a method and a device in which the NOx content of the flue gas from a gas turbine of a combined-cycle power plant is reduced by, on one hand, achieving a reduction by flue gas recirculation and, on the other hand, arranging a dry 3-way catalytic converter in the flue gas stream to the waste heat steam generator. Although this type of catalytic converter avoids the use of NH3 and the associated problem of contaminating the flue gases with NH3 (“NH3 slipping”), it is however extremely expensive due to the catalyst material used.